The reason that you are using the IP address 192.168.10.128 and not 192.168.10.129 is because you are asked to stop Accounting users accessing the HR server attached to Router B.

The accounting users are on the subnet 192.168.10.128/27 (255.255.255.224) and 192.168.10.129 is the default gateway for that subnet. The range of the accounting subnet is 192.168.10.128 --> Subnet Number

If you only denied the gateway address, then only traffic which originated on the router would be blocked. The traffic from the accounting host machines (192.168.10.130 to 192.168.10.158) would all pass through the ACL to the HR server.

Whenever u see an address with a CIDR block prefix, always quickly mentally find out the subnet, broadcast, gateway, host range. Write it down and then work the problem. Then you will easily see the solution.